.TH std::formatted_size 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::formatted_size \- std::formatted_size

.SH Synopsis
   Defined in header <format>
   template< class... Args >
   std::size_t formatted_size( std::format_string<Args...> fmt,       \fB(1)\fP \fI(since C++20)\fP
   Args&&... args );
   template< class... Args >
   std::size_t formatted_size( std::wformat_string<Args...> fmt,      \fB(2)\fP \fI(since C++20)\fP
   Args&&... args );
   template< class... Args >

   std::size_t formatted_size( const std::locale& loc,                \fB(3)\fP \fI(since C++20)\fP

                               std::format_string<Args...> fmt,
   Args&&... args );
   template< class... Args >

   std::size_t formatted_size( const std::locale& loc,                \fB(4)\fP \fI(since C++20)\fP

                               std::wformat_string<Args...> fmt,
   Args&&... args );

   Determine the total number of characters in the formatted string by formatting args
   according to the format string fmt. If present, loc is used for locale-specific
   formatting.

   The behavior is undefined if std::formatter<std::remove_cvref_t<Ti>, CharT> does not
   meet the BasicFormatter requirements for any Ti in Args.

.SH Parameters

             an object that represents the format string. The format string consists of
               * ordinary characters (except { and }), which are copied unchanged to
                 the output,
               * escape sequences {{ and }}, which are replaced with { and }
                 respectively in the output, and
               * replacement fields.

             Each replacement field has the following format:

             { arg-id (optional) }               \fB(1)\fP
             { arg-id (optional) : format-spec } \fB(2)\fP

             1) replacement field without a format specification
             2) replacement field with a format specification

                           specifies the index of the argument in args whose value is
                           to be used for formatting; if it is omitted, the arguments
                           are used in order.
             arg-id      -
                           The arg-id s in a format string must all be present or all
                           be omitted. Mixing manual and automatic indexing is an
                           error.
   fmt     -               the format specification defined by the std::formatter
             format-spec - specialization for the corresponding argument. Cannot start
                           with }.

               * For basic types and standard string types, the format specification is
                 interpreted as standard format specification.
               * For chrono types, the format specification is interpreted as chrono
                 format specification.

               * For range types, the format specification is interpreted
                 as range format specification.
               * For std::pair and std::tuple, the format specification
                 is interpreted as tuple format specification.
               * For std::thread::id and std::stacktrace_entry, see       (since C++23)
                 thread id format specification and stacktrace entry
                 format specification.
               * For std::basic_stacktrace, no format specifier is
                 allowed.

               * For std::filesystem::path, see path format               (since C++26)
                 specification.

               * For other formattable types, the format specification is determined by
                 user-defined formatter specializations.
   args... - arguments to be formatted
   loc     - std::locale used for locale-specific formatting

.SH Return value

   The total number of characters in the formatted string.

.SH Exceptions

   Propagates any exception thrown by formatter.

.SH Example


// Run this code

 #include <format>
 #include <iostream>
 #include <vector>
 #include <string_view>

 int main()
 {
     using namespace std::literals::string_view_literals;

     constexpr auto fmt_str { "Hubble's H{0} {1} {2:*^4} miles/sec/mpc."sv };
     constexpr auto sub_zero { "₀"sv };  // { "\\u2080"sv } => { 0xe2, 0x82, 0x80 };
     constexpr auto aprox_equ { "≅"sv }; // { "\\u2245"sv } => { 0xe2, 0x89, 0x85 };
     constexpr int Ho { 42 }; // H₀


     const auto min_buffer_size = std::formatted_size(fmt_str, sub_zero, aprox_equ, Ho);

     std::cout << "Min buffer size = " << min_buffer_size << '\\n';

     // Use std::vector as dynamic buffer. Note: buffer does not include the trailing '\\0'.
     std::vector<char> buffer(min_buffer_size);

     std::format_to_n(buffer.data(), buffer.size(), fmt_str, sub_zero, aprox_equ, Ho);

     std::cout << "Buffer: \\"" << std::string_view{buffer.data(), min_buffer_size} << "\\"\\n";

     // Or we can print the buffer directly by adding the trailing '\\0'.
     buffer.push_back('\\0');
     std::cout << "Buffer: \\"" << buffer.data() << "\\"\\n";
 }

.SH Output:

 Min buffer size = 37
 Buffer: "Hubble's H₀ ≅ *42* miles/sec/mpc."
 Buffer: "Hubble's H₀ ≅ *42* miles/sec/mpc."

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to      Behavior as published               Correct behavior
   P2216R3 C++20      throws std::format_error for     invalid format string results in
                      invalid format string            compile-time error
                      objects that are neither
   P2418R2 C++20      const-usable nor copyable        allow formatting these objects
                      (such as generator-like objects)
                      are not formattable
   P2508R1 C++20      there's no user-visible name for the name basic_format_string is
                      this facility                    exposed

.SH See also

   format_to   writes out formatted representation of its arguments through an output
   (C++20)     iterator
               \fI(function template)\fP
   format_to_n writes out formatted representation of its arguments through an output
   (C++20)     iterator, not exceeding specified size
               \fI(function template)\fP
